1. Field of the Invention
The present invention relates to a method for fabricating a field oxide in a semiconductor device, and more particularly to a method for fabricating a field oxide in a semiconductor device which can alleviate stresses of the field oxide and enhance ability to isolate the semiconductor device.
2. Description of the Prior Art
As generally known in the art, an isolation layer in a semiconductor device, which isolates an element from another element, takes on more and more important roles as the degree of integration of the semiconductor device increases.
Conventionally, although an LOCOS (Local Oxidation of Silicon) process which utilizes oxidation inhibiting layer such as nitride layer in order to oxidize an isolation region, thereby forming an isolation layer, is used to fabricate an isolation layer in a semiconductor device, however, limitations exist in this conventional method as the degree of integration increases.
Therefore, an STI (Shallow Trench Isolation) process is suggested to solve limitations existed in this conventional art. As the STI process can solve problems occurring in the LOCOS process and can be utilized to a device with considerably high degree of integration, the STI process is nowadays the most widely utilized isolation process.
However, limitations also exist in the STI process due to increase of the degree of integration of the semiconductor device. For example, although an inner part of a trench should be filled up with an oxide layer formed by a CVD (Chemical Vapor deposition) process, it is not easy to fill up the trench with the oxide film when there is a significant decrease of the width of the trench, and therefore limitations exist in filling up the trench to cope with present technological requirements.
Further, as the width of the field oxide of the trench decreases, a probability that the strength of an electric field produced from a cell may affect neighboring cells increases. Therefore, as the semiconductor device is scaled down, a probability that side walls of the field oxide may be under an inversion condition due to the electric field produced from the neighboring cells increases, thereby degrading the characteristics of the semiconductor device.
Also, the influences of the electric field produced from the peripheral cells to a neighboring MOSFET are liable to increase, thereby increasing the possibility of abnormal operation of the semiconductor device.
To solve these problems occurring in the conventional trench isolation process, another conventional method for depositing the inner part of the trench with polysilicon is disclosed in “Tech. Digest of IEDM” by Yongjik PARK and Kinam KIM, pp. 391–394(2001).
However, in the case of depositing polysilicon on a narrow area, such as the trench, too much stress is produced in the neighboring devices when grains grow in a subsequent thermal process, thereby deteriorating refresh characteristics of the semiconductor device.